Multiple Exciton Research Articles

MXene-motivated accelerated charge transfer over TMCs quantum dots for solar-powered photoreduction catalysis

Transition metal chalcogenides quantum dots (TMCs QDs) demonstrate emerging potential in solar energy conversion owing to marvelous physicochemical properties, such as quantum size effect, large extinction coefficient, and multiple excitons generation. Nevertheless, TMCs QDs as photosensitizers are unstable owing to high surface energy, and moreover, substantial trap states of TMCs QDs annihilate the photo-induced charge carriers, thus resulting in ultra-short charge lifetime and inferior photoactivities. Herein, 0D/2D TMCs QDs/MXene heterostructures were designed by a ligand-initiated electrostatic self-assembly strategy, wherein positively charged TMCs QDs were anchored on the negatively charged ultrathin Ti3C2Tx nanosheets (NSs) framework, which promotes the anisotropic unidirectional electron transport from TMCs QDs to MXene NSs. MXene functions as an effective charge mediator to decrease the charge recombination and prolong the charge lifetime for photocatalytic hydrogen generation and anaerobic reduction of nitroaromatics to amino compounds under visible light irradiation. Our work would shed new insights on the role of MXene in tuning the directional charge transport for solar energy conversion.

Aspect ratio dependent air stability of PbSe nanorods and photovoltaic applications.

Development of unique strategies to overcome Shockley–Queisser (SQ) limit in solar cells has gained a great deal of interest. Multiple exciton generation (MEG) process has been considered as one of the best approaches to the SQ limitation. In this respect, PbSe quantum dots (QDs) and nanorods (NRs) have been regarded as promising solar energy harvesting materials owing to their noticeable MEG yields. Although air stability has been regarded as one of the main disadvantage of PbSe QDs, no study has pointed out to the air sensitivity of PbSe NRs yet. Here, we reveal the effect of aspect ratio on air sensitivity and optical properties of PbSe NRs and discover that NRs with higher aspect ratios are more air stable, attributed to the reduced density of NR ends with air sensitive {100} facets. Furthermore, a band offset was created by utilization of tetrabutylammonium iodide and 1,2-ethanedithiol ligands in cell designs. We found that solar cells based on pristine PbSe NRs are limited by low open circuit voltages due to leakage current pathways. On the other hand, modified cells comprising light absorbing layers prepared by blending NRs and QDs and hole transporting QD layer exhibit a 10-fold improvement in solar cell efficiency.

Quantized exciton-exciton annihilation in monolayer WS2 on SrTiO3 substrate with atomically flat terraces

Monolayer materials are strongly affected by their potential fluctuations, which are induced by an intrinsic corrugation or the surface roughness of the substrate. We compare the effective exciton-exciton annihilation (EEA) rate constants of monolayer WS2 on substrates with different surface topographies. We show that monolayer WS2 on the substrate with atomically flat terraces displays small effective EEA rate constant deviating from the overall tendency and multiple exciton decay components, which cannot be accounted for by a conventional EEA model. To obtain a correct description, we use a quantized EEA model. The intrinsic EEA rate constant for the flat-terrace substrates determined by this new model is comparable to that of hBN-encapsulated monolayer WS2.

Women in the Singlet Fission World: Pearls in a Semi-Open Shell.

Singlet fission, a multiple exciton generation process, can revolutionize existing solar cell technologies. Offering the possibility to double photocurrent, the process has become a focal point for physicists, chemists, software developers, and engineers. The following review is dedicated to the female investigators, predominantly theorists, who have contributed to the field of singlet fission. We highlight their most significant advances in the subject, from deciphering the mechanism of the process to designing coveted singlet fission materials.

Multiple spin-orbit excitons in \u03b1-RuCl3 from bulk to atomically thin layers

The van der Waals Kitaev magnet α-RuCl3 has recently garnered considerable attention due to its possible realization of topological spin liquids. Combining Raman spectroscopy with numerical calculations, we report here the thickness dependence of electronic structure and ensuing low-energy excitations for exfoliated α-RuCl3. We observe two pronounced peaks at A1 = 249 meV and A2 = 454 meV, which are assigned to single and double spin-orbit (SO) excitons, respectively. Our numerical calculations support this interpretation by reproducing their spectral energy and shape with the electronic parameters: SO coupling λ = 140 meV, Hund’s coupling JH = 350 meV, and on-site Coulomb interaction U = 2.35 eV. The multiple SO excitons persist down to a single layer, whereas their peaks shift slightly to lower energy. For frequencies below 350 cm−1, both a magnetic continuum and phonons show noticeable thickness dependence. These results demonstrate that a SO entangled jeff = 1/2 picture remains valid in a monolayer limit despite the presence of lattice distortions.

Singlet fission and tandem solar cells reduce thermal degradation and enhance lifespan

AbstractThe economic value of a photovoltaic installation depends upon both its lifespan and power conversion efficiency. Progress toward the latter includes mechanisms to circumvent the Shockley‐Queisser limit, such as tandem designs and multiple exciton generation (MEG). Here we explain how both silicon tandem and MEG‐enhanced silicon cell architectures result in lower cell operating temperatures, increasing the device lifetime compared to standard c‐Si cells. Also demonstrated are further advantages from MEG enhanced silicon cells: (i) the device architecture can completely circumvent the need for current‐matching; and (ii) upon degradation, tetracene, a candidate singlet fission (a form of MEG) material, is transparent to the solar spectrum. The combination of (i) and (ii) mean that the primary silicon device will continue to operate with reasonable efficiency even if the singlet fission layer degrades. The lifespan advantages of singlet fission enhanced silicon cells, from a module perspective, are compared favorably alongside the highly regarded perovskite/silicon tandem and conventional c‐Si modules.

Quantum Dots for Photovoltaics: A Tale of Two Materials

AbstractQuantum dot (QD) solar cells, benefiting from unique quantum confinement effects and multiple exciton generation, have attracted great research attention in the past decades. Before 2016, research efforts were mainly devoted to solar cells comprising lead chalcogenide QDs, while lead halide perovskite QDs have recently emerged as a rising star in the field. This review aims to compare similarities and differences between lead chalcogenide and lead halide perovskite QDs for photovoltaic applications. The fundamental physical properties of these two types of nanomaterials and their state‐of‐the‐art photovoltaic devices are summarized, with a focus on ligand and device engineering. Furthermore, a special section is devoted to the stability issue that often hinders photovoltaic technologies. Finally, future development in tandem devices, challenges associated with large‐size cell fabrication and lead toxicity, and potential mitigation solutions are discussed.

Microscopic reversibility demands lower open circuit voltage in multiple exciton generation solar cells

Multiple exciton generation (MEG) increases the short circuit current of solar cells and is, therefore, often cited as a candidate scheme for surpassing the efficiency limit of single junction solar cells. Conventionally, limiting efficiencies for MEG solar cells have been calculated using quasi-equilibrium models that implicitly assume an effective separation of timescales between different processes. We show here that this separation of timescales is not possible for MEG solar cells, with Auger recombination, the inverse process to multi-exciton generation, needing to be considered explicitly in the efficiency limits of an MEG solar cell. We assess the impact of Auger recombination using a non-equilibrium model of a quantum dot solar cell that satisfies microscopic reversibility and can approximate experimental external quantum efficiency (EQE) curves of MEG solar cells. Recombination—both Auger and radiative—is treated in a quasi-equilibrium approach, which can be justified with a clear model for the separation of timescales. A key insight of this model is that the achievable voltage of the device, and hence the solar energy conversion efficiency, depends on the absolute values of the impact ionization rate and the rate at which electrons lose energy through phonon scattering. By contrast, the EQE profile at short circuit depends only on the ratio of these two rates. This shows that the potential of certain MEG solar cell approaches cannot be assessed from EQE improvements alone, which highlights the importance of considering non-equilibrium processes in models of solar energy conversion devices.

Quantum Dots‐Based Photoelectrochemical Hydrogen Evolution from Water Splitting

AbstractSolar‐driven photoelectrochemical (PEC) hydrogen evolution is a promising and sustainable approach to convert solar energy into a fuel that can be stored. Semiconductor quantum dots (QDs) are increasingly used in PEC devices due to their broad composition/size/shape tunable absorption spectrum (from ultraviolet to near‐infrared, with significant overlap with the solar spectrum). Despite significant efforts and recent progress, several major challenges remain unresolved in this fast‐developing field. Here, the latest progress in tailoring the materials, structure, and performance of QDs‐based PEC H2 generation, including photoanodes, photocathodes, and tandem PEC systems, is summarized. In particular, recent strategies developed for PEC H2 generation are critically analyzed. Specific features of QDs (e.g., size/shape/composition‐tunable absorption band edge arising from quantum confinement, ease of fabrication through chemical approaches, and multiple exciton generation), charge generation, and charge transfer of photoelectrodes and their implications on the performance of PEC devices are discussed. Future challenges and opportunities working, toward high‐efficiency and stable QDs‐based PEC applications are discussed in the conclusion.

Flexible all fluorescence white organic light emitting device with over 22% EQE by stepped reverse intersystem crossing channels based on ternary exciplex

Improvement of the utilization of triplet excitons for thermally activated delayed fluorescence (TADF) emitter-based white organic light emitting devices (WOLEDs) is a key scientific challenge. In this study, a new strategy with stepped reverse intersystem crossing (RISC) channels induced by new ternary exciplex was proposed to enhance up-conversion of non-radiative triplet excitons. A flexible TADF WOLED with external quantum efficiency (EQE) of 22.7% and a power efficiency of 62.5 lmW −1 was demonstrated. At 1000 cdm −2 , the EQE still remained at 21.4%, showing low efficiency roll-off of 5.7%. It is attributed to the reduced non-radiative triplet excitons stack, which suppressed the triplet–triplet quenching. Moreover, the new ternary exciplex-based WOLED system mCP: DMAC-DPS:PO-T2T:4CZPNPh exhibited rate constant of RISC process of 2.5 × 10 6 s −1 , and about 2 times photoluminescence quantum yield over binary exciplex. The high efficiency effectively demonstrated that the proposed multiple triplet excitons capture process is an effective strategy to improve the utilization of triplet excitons. Moreover, the novel strategy can be a promising approach for the further development of WOLED. ● New designed ternary exciplex was proposed and exhibit high efficiency,high photoluminescence quantum yield (PLQY). ● Novel strategy with stepped reverse intersystem crossing channels was introduced to improve the triplet excitons utilization and the PLQY of fluorescence based OLED. ● The flexible all fluorescence WOLED achieved maximum external quantum efficiency, power efficiency, current efficiency of 22.7%, 62.5 lm/W, 57.8 cd/A. At 1000 cd/m 2 , the external quantum efficiency is still remain 21.4%, exhibit superior efficiency stability.

Twist Angle-Dependent Interlayer Exciton Lifetimes in van der Waals Heterostructures.

In van der Waals (vdW) heterostructures formed by stacking two monolayers of transition metal dichalcogenides, multiple exciton resonances with highly tunable properties are formed and subject to both vertical and lateral confinement. We investigate how a unique control knob, the twist angle between the two monolayers, can be used to control the exciton dynamics. We observe that the interlayer exciton lifetimes in MoSe_{2}/WSe_{2} twisted bilayers (TBLs) change by one order of magnitude when the twist angle is varied from 1° to 3.5°. Using a low-energy continuum model, we theoretically separate two leading mechanisms that influence interlayer exciton radiative lifetimes. The shift to indirect transitions in the momentum space with an increasing twist angle and the energy modulation from the moiré potential both have a significant impact on interlayer exciton lifetimes. We further predict distinct temperature dependence of interlayer exciton lifetimes in TBLs with different twist angles, which is partially validated by experiments. While many recent studies have highlighted how the twist angle in a vdW TBL can be used to engineer the ground states and quantum phases due to many-body interaction, our studies explore its role in controlling the dynamics of optically excited states, thus, expanding the conceptual applications of "twistronics".

Exciton-Photon Interactions in Semiconductor Nanocrystals: Radiative Transitions, Non-Radiative Processes and Environment Effects

In this review, we discuss several fundamental processes taking place in semiconductor nanocrystals (quantum dots (QDs)) when their electron subsystem interacts with electromagnetic (EM) radiation. The physical phenomena of light emission and EM energy transfer from a QD exciton to other electronic systems such as neighbouring nanocrystals and polarisable 3D (semi-infinite dielectric or metal) and 2D (graphene) materials are considered. In particular, emission decay and FRET rates near a plane interface between two dielectrics or a dielectric and a metal are discussed and their dependence upon relevant parameters is demonstrated. The cases of direct (II–VI) and indirect (silicon) band gap semiconductors are compared. We cover the relevant non-radiative mechanisms such as the Auger process, electron capture on dangling bonds and interaction with phonons. Some further effects, such as multiple exciton generation, are also discussed. The emphasis is on explaining the underlying physics and illustrating it with calculated and experimental results in a comprehensive, tutorial manner.

Study on the influence of embedded structure of carbon quantum dots of the organic solar cells with the territory active layer structure of P3HT: PC61BM: CQDs

In this work, carbon quantum dots (CQDs) were inserted into the poly(3-hexylthiophene):(6,6)-phenyl-C61 butyric acid acrylic (P3HT:PC61BM) system as the third component. Therefore, organic solar cells (OSCs) based on the ternary active layer structure of P3HT:PC61BM:CQDs were prepared. The lattice distance of CQDs was performed using transmission electron microscopy (TEM). The excellent fluorescence emission and light absorption of CQDs were characterized by photoluminescence (PL) and UV–vis absorption spectra. The characterization results of atomic force microscope (AFM) and electrochemical impedance spectroscopy (EIS) indicate that the introduction of CQDs in the active layer can improve the performance of the device but not significantly change the properties of the film. The device based on P3HT:PC61BM:CQDs ternary structure obtains a power conversion efficiency (PCE) of 3.32%, which has a 32.8% increase compared to the reference device (2.50%). Besides, the best short-circuit current density attends to 18.9 mA/cm2, which is increased by 25% compared to the reference device. The improvement is attributed to the photoluminescence effect and the multiple exciton generation (MEG) effect of CQDs. This work provides a new method to improve the performance of organic solar cells.

Multiple exciton generation in isolated and interacting silicon nanocrystals.

An important challenge in the field of renewable energy is the development of novel nanostructured solar cell devices which implement low-dimensional materials to overcome the limits of traditional photovoltaic systems. For optimal energy conversion in photovoltaic devices, one important requirement is that the full energy of the solar spectrum is effectively used. In this context, the possibility of exploiting features and functionalities induced by the reduced dimensionality of the nanocrystalline phase, in particular by the quantum confinement of the electronic density, can lead to a better use of the carrier excess energy and thus to an increment of the thermodynamic conversion efficiency of the system. Carrier multiplication, i.e. the generation of multiple electron-hole pairs after absorption of one single high-energy photon (with energy at least twice the energy gap of the system), can be exploited to maximize cell performance, promoting a net reduction of loss mechanisms. Over the past fifteen years, carrier multiplication has been recorded in a large variety of semiconductor nanocrystals and other nanostructures. Owing to the role of silicon in solar cell applications, the mission of this review is to summarize the progress in this fascinating research field considering carrier multiplication in Si-based low-dimensional systems, in particular Si nanocrystals, both from the experimental and theoretical point of view, with special attention given to the results obtained by ab initio calculations.

Magnetic and power tuning of spin-asymmetric multiple excitons in a GaAs quantum well

Abstract A comprehensive study of excitonic complexes in high-quality GaAs quantum wells has been performed. The polarization-resolved photoluminescence results show exciton, biexciton, and trion emissions tuned by temperature, excitation power, and by an external magnetic field. Excitons and biexcitons show important differences in their dependence on the external field. While the Zeeman splitting of biexcitons depends monotonically on the magnetic field, with a nearly constant g-factor, the behaviour of the exciton energy splitting is nonmonotonic and includes an inversion of sign as a function of the magnetic field. More importantly, we observe that a trion resonance emerges at a finite magnetic field, and only appears at low laser excitation power and for σ+ polarization emission. The non-trivial dependence of the energy levels on the external magnetic field, together with the polarization selective trion emission, leads to an intricate exciton and trion dynamics, which can be reproduced by a set of coupled rate equations. Based on this theoretical approach, we show that a fast spin-flip process can lead to the spin-asymmetric emission, which reveals a contrasting spin-dynamics of excitonic complexes ruled by their charge.

Auger Recombination and Multiple Exciton Generation in Colloidal Two-Dimensional Perovskite Nanoplatelets: Implications for Light-Emitting Devices

Improving the understanding of multiple exciton interactions and dynamics in semiconductor nanostructures is mandatory for their successful use as photoactive materials in light convertors such as electroluminescent diodes, lasers, or single-photon sources. Here high-fluence and high-energy excitation effects are investigated in strongly confined two-dimensional (2D) lead iodide perovskite nanoplatelets (NPLs) using time-resolved photoluminescence and femtosecond transient absorption spectroscopy. Nonradiative Auger recombination (AR) is the dominant pathway for multiexciton recombination. Its dynamics are found to be subquadratic with the exciton density. Indeed, because of the limited exciton wave-function delocalization length, AR is limited by exciton diffusion in the 2D plane at moderate excitation fluence and takes place in several hundreds of picoseconds, with typical recombination rates on the order of 10–2 cm2/s. At high excitation fluence leading to an average interexciton distance comparable with the exciton delocalization length, the measured “intrinsic” AR time is faster than 10 ps and independent of the NPL composition. The strong dependence of the AR rate on the interexciton distance allows us to identify the recombination resulting from multiple exciton generation, involving the reaction of “geminate biexcitons”, upon excitation at low fluence with high-energy photons.

Hybrid white light emitting devices (HWLEDs) from organic polymer and PbS nanocrystals by multiple excitons

Abstract Hybrid white light-emitting devices (HWLEDs) were fabricated using FTO/PEDOT: PSS/PbS/Alq3/Ni system and synthesized by phase separation process. In the present study, the multiple excitons generation in lead sulfide (PbS) NCs, which is characteristic of PbS NCs, was used to induce an effective and regulated energy transfer to an HWLED. The HWLED consisted of three layers successively deposited on FTO glass substrate; the first layer consisted of poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) blended with polymethyl methacrylate (PMMA) organic polymer in the 1:1 ratio, while the second layer consisted of PbS NCs. Finally, above the layer of the PbS NCs, Tris (8-hydroxyquinoline) aluminum (Alq3) layer was deposited. The white light was generated with quite a good efficiency due to the confinement effect that makes the energy gap greater. The characteristics of the current-voltage (I-V) indicate acceptable conditions for the generation of white light by multiple excitons. It was found that the emission levels able to produce white luminescence, classified based on the coordinate system of chromaticity (CIE 1931), are x = 0.31, y = 0.33 while the correlated color temperature (CCT) is about 6250 K. The HWLEDs made from PbS NCs with hole injection from the organic polymer (PEDOT: PSS with PMMA), and electron injection from organic molecules (Alq3) are capable of white light generation.

The effect of activated carbon additives on lead sulphide thin film for solar cell applications

A photovoltaic device with an efficiency that could break the theoretical limit, exceeding ~60% is the focus in the research field in recent years. The efficiency of three important processes in a photovoltaic device need to be ensured to materialize the goal i.e., electron excitations, injections and regenerations. A multiple exciton generation (MEG) mechanism has been proven to increase the photovoltaic conversion efficiency - achievable via usage of small size lead chalcogenides as main light absorber of the photovoltaic device. An efficient electron injection in an excitonic solar cell could be achieved upon fulfilment of the following factors i.e., (i) LUMOfluorophore>CBphotoelectrode, and (ii) small offset between the LUMOfluorophore and CBphotoelectrode. The opto-electronic properties of lead chalcogenide are tuneable based on its size and morphology. Therefore, a synthesis method that could control the size and morphology of the yielded lead chalcogenide plays an important role. This research investigated the effect of additional activated carbon (AC) to the yielded PbS using vacuum thermal evaporator method. The PbS thin films were fabricated with addition of AC with different surface areas i.e., 80 m2/g, 650 m2/g and 1560 m2/g using thermal evaporator at vacuum pressure of 1.0 × 10−5 Torr. The surface area of the ACs was determined using Micromeritics ASAP 2020 BET (Brunauer-Emmett-Teller). The morphology, elemental analysis, crystal structure, opto-electronic, electron injection efficiency and electrical conductivity of the PbS thin film was characterized using Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive X-Ray Spectrometer (EDX), X-Ray Diffractometer (XRD), absorption spectrometer, photoluminescence spectrometer (PL), and Bridge Technology 4-point probes (4PP) respectively. The excited and ground states of the PbS, and redox potential of ionLic PMII electrolyte were determined using quantum chemical calculations at b3lyp/lanl2dz level of theory. Three important observations have been made i.e., (i) addition of AC with the PbS reactants affects the yielded morphology of PbS thin film, (ii) bare PbS/TiO2 device structure offers electron injection efficiency as high as 97% from the PbS to TiO2, and (iii) the bare PbS/TiO2 device structure would offer maximum VOC of ca. 1.7 V, however need to be paired with an electrolyte that possess oxidation potential of ca. −6.5 eV.

Electrochemical Epitaxial Grown PbS Nanorods Array on Graphene Film for High‐Performance Photodetector

AbstractPbS based low dimensional heterostructures have shown promising IR photodetector applications, arising from the multiple exciton effect. However, owing to the difficulty in controlling the size, shape, nucleation, and growth of PbS nanorods, it remains a significant challenge to construct PbS nanorods array heterostructures with precisely controlled orientation, morphology, dimensionality, and spatial distribution of each component. Here, an electrochemical atomic layer epitaxial growth method is reported to prepare the PbS nanorods array‐graphene heterostructures by engineering the structures, sizes, dimensionalities, and morphologies lattice parameters of PbS. In these heterostructures, PbS nanorods selectively grow along (200) orientation on graphene film by adjusting the underpotential deposition. The high‐oriented PbS nanorods‐graphene heterostructure's growth dynamics are confirmed by comparing the extreme polarization of graphene, resulting in controllable growth orientation and nucleation. Furthermore, the PbS nanorods‐graphene heterostructure photodetector presents an extraordinary photoresponsivity in exceeding 10.4 A W−1 and figure‐of‐merit detection D* ≈ 2.98 × 109 Jones is demonstrated in 2.7 µm at room temperature. The selective epitaxial growth of PbS nanorods opens the way to synthesize different hierarchical heterostructures with tunable architectures and dimensionalities, which can process various promising applications.

Finite element analysis of the effect of wetting layer on the electronic eigenstates of InP/InGaP pyramidal quantum dots solar cell

The efficiency of quantum dot (QD) to convert sunlight to electricity is based on the principle of multiple exciton generation (MEG), such that a single photon produces more than one electron-hole pair. The zero-dimensional structure of QD leads to spatial confinement of electronic energies. The energy levels of electrons in QDs depends on their size – the smaller the quantum dot, the greater the amount of energy required to excite electrons to the next level. This allows quantum dots to be tuned to absorb different wavelengths of light just by changing their size. The tuning of band in Quantum dot (QD) structures offers potential applications in the field of photovoltaics and optoelectronic devices. In this paper, we have presented the finite element method simulation results of solar cell based on InP pyramidal QDs in an InGaP matrix. We have found the ground state energies using time independent Schrödinger equation (SE) with effective mass approximation. We show that ground state energies are affected by the dimension of the QDs as well as by the presence of a quantum well wetting layer associated with the QDs.